The invention relates in general to cross-connects which have interfaces to an external communications system, said interfaces being controlled by an internal control circuit in the cross-connect. In particular the invention relates to devices of said nature, in which the operation of the control circuit depends on whether the connections with the external communications system are functioning faultlessly.
FIG. 1a shows a base station network in a cellular radio system, comprising a base station controller 100 (BSC) and a plurality of base transceiver stations 101 (BTS). The base station controller 100 is further connected to a mobile switching center, which is not shown. To enable transmission of data between them, the base station controller 100 and base transceiver stations 101 are interlinked through a plurality of connections which constitute a so-called transmission system in the base station network. The standards for a cellular radio system such as the Global System for Mobile Telecommunications (GSM), for example, usually do not specify the transmission method to be used in the base station network, except for defining the functions that the transmission method has to be capable to realize. In GSM, the interface between two base transceiver stations or a base transceiver station and the base station controller, as defined in the standards, is called the Abis interface. The transmission method may comprise e.g. a 2-Mbit/s or 1.5-Mbit/s PCM connection (Pulse Coded Modulation; ITU-T G.703 and G.704), SDH connection (Synchronous Digital Hierarchy; ITU-T G.774.03), ATM connection (Asynchronous Transfer Mode; ETS 300 371), ISDN connection (Integrated Services Digital Network), or a HDSL connection (High Density Digital Subscriber Line). The physical connection may comprise an ordinary copper wire, optical cable or a microwave radio link.
In the base transceiver stations and base station controller of the system depicted in FIG. 1a connection to the transmission system is realized through a cross-connect 102. A cross-connect 102 in a base transceiver station may comprise one or more transmission units (TRU). Cross-connecting means that the incoming data, which are arranged in frames, can be connected to the outgoing direction in the cross-connecting device such that the location of the data bits in the frames can be altered. The base transceiver station cross-connect xe2x80x9cdropsxe2x80x9d certain bits and time slots in the transmission system frame to the base transceiver station, i.e. directs data concerning that particular base transceiver station, which arrive in certain time slots, to the base transceiver station and, on the other hand, associates the data leaving the base transceiver station in the direction of the base station controller with certain time slots allocated to that base transceiver station. The cross-connect may also perform summing, multiplication or other operations on the incoming data before the data are connected to the outgoing direction. When the cross-connect is placed either in the same equipment rack with the base transceiver station or in its immediate vicinity, the base transceiver station constitutes a compact unit and the base station network can be easily modified and expanded.
The transmission capacity allocated to one base transceiver station depends on how many TRX (Transmit/Receive) units 103 it contains. The TRXs constitute a radio interface to terminal equipment 104, and the number of TRX units determines how many simultaneous speech or data connections the base transceiver station can handle. Different parts of the base station network may also require different amounts of transmission capacity depending on the base station network topology. In a tree-like base station network the highest capacity is required of connections near to the base station controller.
At its simplest a transmission system comprises a so-called point-to-point connection where a given GSM base transceiver station communicates directly with the base station controller and through the latter with a switching center. However, in the case of a 2-Mbit/s PCM, for example, the traffic capacity required by a base transceiver station having one TRX is quite small compared to the whole transmission band. Typically, two and a half time slots in a PCM frame (6 to 8 voice channels and signalling), or 160 kbit/s, are reserved for one TRX. Therefore, a point-to-point connection often wastes capacity and becomes expensive. On the other hand, the use of existing ISDN connections for point-to-point connections may be an alluring idea. Network back-up can be realized using redundant point-to-point connections.
The transmission band can be utilized more efficiently by chaining base transceiver stations (so-called multidrop chain structure). In the chain, several base transceiver stations share, on a time division basis, the same transmission medium, thus better utilizing the connection capacity. Thus the integrated cross-connecting function in the base transceiver station really becomes useful as the time slot arrangements can be made within the base transceiver station.
Loop networks are used for network duplication. Base transceiver stations are looped together so that there exists at all times a transmission connection in both directions of the loop from each base transceiver station to the BSC. Normally, one of the connections is active. Network monitoring is realized using status bits, or so-called called pilot bits, which each base transceiver station sends in both transmission directions in the loop. A change in the state of a pilot bit indicates a network fault, at which point cross-connects in base transceiver stations switch over to the back-up connection. Network synchronization data are also sent using status bits of their own. A switch-over as quick as possible enables network operation without disconnected calls even in fault situations. A GSM call can tolerate a 500-ms break in the transmission connection without disconnecting the call proper.
FIG. 1b shows a prior-art cross-connect in a GSM base transceiver station. It has two separate transmission units 110 and 111. Both transmission units have an xe2x80x9cout-boundxe2x80x9d Abis interface according to the GSM standards, i.e. an interface to either the base station controller or another base transceiver station (not shown). In addition, both transmission units have an administrative connection to the base station controller. One of the transmission units is also connected to the internal data bus in the base transceiver station which is used in sending the downlink data associated with the voice and signalling connections handled by the base transceiver station to the TRX units (not shown) of the base transceiver station, and, correspondingly, the uplink data from the TRX units to the base station controller. In the prior-art implementation the transmission units 110 and 111 in the cross-connect are wholly separate and they both have internal cross-connecting buses of their own. The transmission units are interconnected through the Abis interface as shown in FIG. 1b. 
In future cellular radio systems the average cell size will be smaller and, hence, the number of cells greater than today so that transmission systems shall be capable of handling more base transceiver stations, and network topologies and cross-connections will be more complex than now. The operator providing the transmission medium will not necessarily be the same as the operator running the cellular radio system, so the latter must be able to realize transmission between base transceiver stations and base station controllers as advantageously and efficiently as possible, using the various transmission possibilities available.
An object of this invention is to provide a base transceiver station cross-connect which responds to transmission system faults quickly, effectively and reliably.
The objects of the invention are achieved by realizing in the cross-connect hardware-based monitoring of transmission system status bits and cross-connect changes on the basis of changes observed in the state of the status bits.
The cross-connect according to the invention comprises a cross-connecting processor, data memory for storing cross-connection data in cyclically recurring elements, and a control memory for storing instructions controlling the cross-connecting. It is characterized in that
it comprises a condition monitoring block arranged so as to selectively read cross-connection data from said data memory and indicate a change in the status of a certain cyclically recurring bit or bit combination, and
said cross-connecting processor is arranged so as to perform, in response to the reading of a certain instruction from said control memory, a certain first switching operation to certain cross-connection data when said condition monitoring block has indicated that said cyclically recurring bit or bit combination is in a predetermined first state, and a certain second switching operation to said cross-connection data when said condition monitoring block has indicated that said cyclically recurring bit or bit combination is in a predetermined second state.
The invention is also directed to a method for realizing conditional cross-connection. The method according to the invention is characterized in that it comprises steps in which
a) a certain portion of the cross-connection data stored in the data memory is read and its status is indicated,
b) a conditional cross-connecting instruction, which contains information on at least two cross-connection data sources, is read from the control memory, and
c) if a predetermined first status was indicated in step a), a certain first switching operation is performed to certain cross-connection data the source of which is specified in the cross-connecting instruction, and
d) if a predetermined second status was indicated in step a), a certain second switching operation is performed to certain cross-connection data the source of which is specified in the cross-connecting instruction.
In the inventional structural solution the cross-connection and the functions required by the transmission connections are implemented modularly by decentralizing them into several parts which in this patent application are called transmission units. Decentralization is to be understood such that a single transmission unit can establish all transmission connections of a base transceiver station but units can be added according to capacity requirements so that they function as a whole. The cross-connection is shared by the transmission units through a parallel bus in the so-called motherboard, which bus interconnects the transmission units and is advantageously duplicated for reliability. From the point of view of base station control the transmission units constitute one controllable whole. Each transmission unit realizes a certain type of standard transmission interface.
As the amount of GSM traffic increases there also emerges a need to have different transmission interfaces in one and the same base transceiver station. Therefore, the new base station solution can use transmission units of many different types. Within a transmission unit, a given first part realizes the transmission interface and converts the received data, which are to be cross-connected, from the format used in the transmission system to the internal format used in the cross-connect. The data are written in that format to the cross-connecting bus interconnecting the transmission units. The other parts of the transmission unit realize advantageously at least cross-connection, unit control, synchronization with other transmission units and interfaces to the base station motherboard. A transmission unit may comprise one or more printed circuit boards. Hereinafter, the term xe2x80x9cspecial partxe2x80x9d refers to parts realizing a transmission interface and the term xe2x80x9ccommon partxe2x80x9d refers to the cross-connecting and bus interface block. In addition to the functions mentioned above, a transmission unit may include other functional blocks, too.
The special part in the transmission unit adapts the cross-connect in the base transceiver station to the base station network""s transmission system, which may be a PCM, HDSL or ISDN system, for example. Advantageously the special part may also comprise adapter circuits for different physical transmission media such as copper wire, optical cable or radio link.
In the common part, all data traveling through the cross-connecting bus are stored in the data memory. One of the blocks in the cross-connecting circuit in the common part is the condition monitoring block which reads, in accordance with pre-set conditions, certain pilot bits from the data stored in the data memory. A change in the state of a pilot bit implies a fault in the transmission system, whereby the state of the status register in the condition monitoring block changes. The instructions controlling the operation of the cross-connecting processor take into account the situations in which the operation depends on the status register bits. As the cross-connecting processor, controlled by an instruction, detects a change in the status register, it carries out the cross-connecting action defined by the instruction in a different manner than if no changes had been detected in the status register.